1. Field of The Invention
This invention relates to computer network latency testing as applied to TCP.
2. Background of the Invention
Tools such as network managers and sniffers are used to determine network latency by transmitting a sample packet or message through a network interface. Typical network interfaces include ethernet interfaces and other connections to a network such as the internet. Typical tools include ping, tracert, and hardware or software tools intended to test interface device response. One software tool used for this purpose is tcpdump.
Traditional tools such as network managers and sniffers report only the quantity of data that has passed an interface in a network. These tools provide little information on global transit times of packets and data across the entire network space of interest to the user.
As mentioned, tcpdump provides a port sniffing function. Tcpdump operates with the interface in promiscuous mode, and allows the user to intercept and display TCP/IP and other packets being transmitted or received over a network to which the computer is attached. Tcpdump is a Unix-based program, although non-Unix implementations have been developed, including WinDump, which is considered to be a port of tcpdump to Windows.
On most operating systems, a user must have superuser privileges to use tcpdump due to its use of promiscuous mode, and due to the fact that various Unix network packet capturing schemes (raw sockets, special devices, etc.) require elevated privileges. The user may optionally apply any number of bpf-based filters to render the output more usable on networks with a high volume of traffic. Tcpdump is used for debugging operations related to network communications, routing and latency. In addition, tcpdump has the capability of monitoring user activity, as a result of its operation as a packet sniffer.
Tcpdump prints out the headers of packets on a network interface that match the Boolean expression. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -b flag, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, only packets that match expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts of:                a. packets “received by filter” (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured—if a filter was specified on the command line, on some OSes it counts packets regardless of whether they were matched by the filter expression, and on other OSes it counts only packets that were matched by the filter expression and were processed by tcp dump);        b. packets “dropped by kernel” (this is the number of packets that were dropped, due to a lack of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that information to applications; if not, it will be reported as 0).        
The general description of the operation of tcpdump can be seen from the man page:
TABLE 1Tcpdump Man Page > man tcpdump tcpdump - dump traffic on a networkSYNOPSIS tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]  [ -C file size ] [ -F file ]  [ -i interface ] [ -m module ] [ -r file ]  [ -s snaplen ] [ -T type ] [ -w file ]  [ -E algo:secret ] [ expression ]DESCRIPTION Tcpdump prints out the headers of packets on a network interface that match the boolean expression. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -b flag, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, only packets that match expression will be processed by tcpdump. Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt charac- ter, typically control-C) or a SIGTERM signal (typically generated with the kill(l) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed. When tcpdump finishes capturing packets, it will report counts of:  packets “received by filter” (the meaning of this  depends on the OS on which you're running tcpdump,  and possibly on the way the OS was configured - if  a filter was specified on the command line, on some  OSes it counts packets regardless of whether they  were matched by the filter expression, and on other  OSes it counts only packets that were matched by  the filter expression and were processed by tcp  dump);  packets “dropped by kernel” (this is the number  of packets that were dropped, due to a lack of  buffer space, by the packet capture mechanism in  the OS on which tcpdump is running, if the OS  reports that information to applications; if not,  it will be reported as 0). On platforms that support the SIGINFO signal, such as most BSDs, it will report those counts when it receives a SIG INFO signal (generated, for example, by typing your “sta- tus” character, typically control-T) and will continue you have special privileges: Under SunOS 3.x or 4.x with NIT or BPF:  You must have read access to /dev/nit or /dev/bpf*. Under Solaris with DLPI:  You must have read/write access to the network  pseudo device, e.g. /dev/le. On at least some  versions of Solaris, however, this is not suffi-  cient to allow tcpdump to capture in promiscuous  mode; on those versions of Solaris, you must be  root, or tcpdump must be installed setuid to root,  in order to capture in promiscuous mode. Under HP-UX with DLPI:  You must be root or tcpdump must be installed  setuid to root. Under IRIX with snoop:  You must be root or tcpdump must be installed  setuid to root. Under Linux:  You must be root or tcpdump must be installed  setuid to root. Under Ultrix and Digital UNIX:  Once the super-user has enabled promiscuous-mode  operation using pfconfig(8), any user may capture  network traffic with tcpdump. Under BSD:  You must have read access to /dev/bpf*. Reading a saved packet file doesn't require special privi- leges.OPTIONS -a Attempt to convert network and broadcast addressesto names. -c Exit after receiving count packets. -C Before writing a raw packet to a savefile, checkwhether the file is currently larger than file_sizeand, if so, close the current savefile and open anew one. Savefiles after the first savefile willhave the name specified with the -w flag, with anumber after it, starting at 2 and continuingupward. The units of file_size are millions ofbytes (1,000,000 bytes, not 1,048,576 bytes). -dd Dump packet-matching code as a C program fragment. -ddd Dump packet-matching code as decimal numbers (pre-ceded with a count). -e Print the link-level header on each dump line. -E Use algo:secret for decrypting IPsec ESP packets.Algorithms may be des-cbc, 3des-cbc, blowfish-cbc,rc3-cbc, castl28-cbc, or none. The default is des-cbc. The ability to decrypt packets is only pre-sent if tcpdump was compiled with cryptographyenabled, secret the ascii text for ESP secret key.We cannot take arbitrary binary value at thismoment. The option assumes RFC2406 ESP, notRFC1827 ESP. The option is only for debugging pur-poses, and the use of this option with truly‘secret’ key is discouraged. By presenting IPsecsecret key onto command line you make it visible toothers, via ps(l) and other occasions. -f Print ‘foreign’ internet addresses numericallyrather than symbolically (this option is intendedto get around serious brain damage in Sun's ypserver -- usually it hangs forever translating non-local internet numbers). -F Use file as input for the filter expression. Anadditional expression given on the command line isignored. -i Listen on interface. If unspecified, tcpdumpsearches the system interface list for the lowestnumbered, configured up interface (excluding loopback). Ties are broken by choosing the earliestmatch.On Linux systems with 2.2 or later kernels, aninterface argument of “any” can be used to cap-ture packets from all interfaces. Note that cap-tures on the “any” device will not be done inpromiscuous mode. -l Make stdout line buffered. Useful if you want tosee the data while capturing it. E.g.,“tcpdump -l | tee dat” or “tcpdump -l >dat & tail -f dat”. -m Load SMI MIB module definitions from file module.This option can be used several times to load sev-eral MIB modules into tcpdump.numbers, etc.) to names. -N Don't print domain name qualification of hostnames. E.g., if you give this flag then tcpdumpwill print “nic” instead of “nic.ddn.mil”. -O Do not run the packet-matching code optimizer.This is useful only if you suspect a bug in theoptimizer. -p Don't put the interface into promiscuous mode.Note that the interface might be in promiscuousmode for some other reason; hence, ‘-p’cannot beused as an abbreviation for ‘ether host {local-hw-addr} or ether broadcast’. -q Quick (quiet?) output. Print less protocol infor-mation so output lines are shorter. -R Assume ESP/AH packets to be based on old specifica-tion (RFCT825 to RFCT829) . If specified, tcpdumpwill not print replay prevention field. Sincethere is no protocol version field in ESP/AH speci-fication, tcpdump cannot deduce the version ofESP/AH protocol. -r Read packets from file (which was created with the-w option) . Standard input is used if file is“-”. -S Print absolute, rather than relative, TCP sequencenumbers. -s Snarf snaplen bytes of data from each packet ratherthan the default of 68 (with SunOS's NIT, the mini-mum is actually 96). 68 bytes is adequate for IP,ICMP, TCP and UDP but may truncate protocol infor-mation from name server and NFS packets (seebelow) . Packets truncated because of a limitedsnapshot are indicated in the output with“[|proto]”, where proto is the name of the proto-col level at which the truncation has occurred.Note that taking larger snapshots both increasesthe amount of time it takes to process packets and,effectively, decreases the amount of packet buffer-ing. This may cause packets to be lost. Youshould limit snaplen to the smallest number thatwill capture the protocol information you're inter-ested in. Setting snaplen to 0 means use therequired length to catch whole packets. -T Force packets selected by “expression” to be inter-cedure Call), rtp (Real-Time Applications proto-col), rtcp (Real-Time Applications control proto-col), snmp (Simple Network Management Protocol),vat (Visual Audio Tool), and wb (distributed WhiteBoard). -t Don't print a timestamp on each dump line. -tt Print an unformatted timestamp on each dump line. -ttt Print a delta (in micro-seconds) between currentand previous line on each dump line. -tttt Print a timestamp in default format proceeded bydate on each dump line. -u Print undecoded NFShandles. -v (Slightly more) verbose output. For example, thetime to live, identification, total length andoptions in an IP packet are printed. Also enablesadditional packet integrity checks such as verifying the IP and ICMP header checksum. -vv Even more verbose output. For example, additionalfields are printed from NFS reply packets, and SMBpackets are fully decoded. -vvv Even more verbose output. For example, telnet SB. . . SE options are printed in full. With -X telnetoptions are printed in hex as well. -w Write the raw packets to file rather than parsingand printing them out. They can later be printedwith the -r option. Standard output is used iffile is “-”. -x Print each packet (minus its link level header) inhex. The smaller of the entire packet or snaplenbytes will be printed. -X When printing hex, print ascii too. Thus if -x isalso set, the packet is printed in hex/ascii. Thisis very handy for analysing new protocols. Even if-x is not also set, some parts of some packets maybe printed in hex/ascii. expression  selects which packets will be dumped. If no  expression is given, all packets on the net will be  dumped. Otherwise, only packets for which expres-  sion is ‘true’ will be dumped.  ber) preceded by one or more qualifiers. There are  three different kinds of qualifier:  type qualifiers say what kind of thing the idname or number refers to. Possible typesare host, net and port. E.g., ‘host foo’,‘net 128.3’, ‘port 20’. If there is no typequalifier, host is assumed.  dir qualifiers specify a particular transferdirection to and/or from id. Possibledirections are src, dst, src or dst and srcand dst. E.g., ‘src foo’, ‘dst net 128.3’,‘src or dst port ftp-data’. If there is nodir qualifier, src or dst is assumed. For‘null’ link layers (i.e. point to point pro-tocols such as slip) the inbound and outbound qualifiers can be used to specify adesired direction.  proto qualifiers restrict the match to a particu-lar protocol. Possible protos are: ether,fddi, tr, ip, ip6, arp, rarp, decnet, tcpand udp. E.g., ‘ether src foo’, ‘arp net128.3’, ‘tcp port 21’. If there is no protoqualifier, all protocols consistent with thetype are assumed. E.g., ‘src foo’ means‘(ip or arp or rarp) src foo’ (except thelatter is not legal syntax), ‘net bar’ means‘(ip or arp or rarp) net bar’ and ‘port 53’means ‘(tcp or udp) port 53’.  [‘fddi’ is actually an alias for ‘ether’; the  parser treats them identically as meaning “the  data link level used on the specified network  interface.” FDDI headers contain Ethernet-like  source and destination addresses, and often contain  Ethernet-like packet types, so you can filter on  these FDDI fields just as with the analogous Ether  net fields. FDDI headers also contain other  fields, but you cannot name them explicitly in a  filter expression.  Similarly, ‘tr’ is an alias for ‘ether’; the previ-  ous paragraph's statements about FDDI headers also  apply to Token Ring headers.]  In addition to the above, there are some special  ‘primitive’ keywords that don't follow the pattern:  gateway, broadcast, less, greater and arithmetic  expressions. All of these are described below.  tives. E.g., ‘host foo and not port ftp and not  port ftp-data’. To save typing, identical quali-  fier lists can be omitted. E.g., ‘tcp dst port ftp  or ftp-data or domain’ is exactly the same as ‘tcp  dst port ftp or tcp dst port ftp-data or tcp dst  port domain’.  Allowable primitives are:  dst host host   True if the IPv4/v6 destination field of the   packet is host, which may be either an   address or a name.  src host host   True if the IPv4/v6 source field of the   packet is host.  host host   True if either the IPv4/v6 source or desti-   nation of the packet is host. Any of the   above host expressions can be prepended with   the keywords, ip, arp, rarp, or ip6 as in:    ip host host   which is equivalent to:    ether proto \ip and host host   If host is a name with multiple IP   addresses, each address will be checked for   a match.  ether dst ehost   True if the ethernet destination address is   ehost. Ehost may be either a name from   /etc/ethers or a number (see ethers(3N) for   numeric format).  ether src ehost   True if the ethernet source address is   ehost.  ether host ehost   True if either the ethernet source or desti-   nation address is ehost.  gateway host   True if the packet used host as a gateway.   I.e., the ethernet source or destination   address was host but neither the IP source   nor the IP destination was host. Host must   be a name and must be found both by the   machine's host-name-to-IP-address resolution   mechanisms (host name file, DNS, NIS, etc.)   etc.) . (An equivalent expression is    ether host ehost and not host host   which can be used with either names or num-   bers for host / ehost.) This syntax does   not work in IPv6-enabled configuration at   this moment.  dst net net   True if the IPv4/v6 destination address of   the packet has a network number of net. Net   may be either a name from /etc/networks or a   network number (see networks(4) for   details).  src net net   True if the IPv4/v6 source address of the   packet has a network number of net.  net net   True if either the IPv4/v6 source or desti-   nation address of the packet has a network   number of net.  net net mask netmask   True if the IP address matches net with the   specific netmask. May be qualified with src   or dst. Note that this syntax is not valid   for IPv6 net.  net net/len   True if the IPv4/v6 address matches net with   a netmask len bits wide. May be qualified   with src or dst.  dst port port   True if the packet is ip/tcp, ip/udp,   ip6/tcp or ip6/udp and has a destination   port value of port. The port can be a num-   ber or a name used in /etc/services (see   tcp(4P) and udp(4P)) . If a name is used,   both the port number and protocol are   checked. If a number or ambiguous name is   used, only the port number is checked (e.g.,   dst port 513 will print both tcp/login traf-   fic and udp/who traffic, and port domain   will print both tcp/domain and udp/domain   traffic).  src port port   True if the packet has a source port value   of port.   True if either the source or destination   port of the packet is port. Any of the   above port expressions can be prepended with   the keywords, tcp or udp, as in:    tcp src port port   which matches only tcp packets whose source   port is port.  less length   True if the packet has a length less than or   equal to length. This is equivalent to:    len <= length.  greater length   True if the packet has a length greater than   or equal to length. This is equivalent to:    len >= length.  ip proto protocol   True if the packet is an IP packet (see   ip(4P)) of protocol type protocol. Protocol   can be a number or one of the names icmp,   icmp6, igmp, igrp, pim, ah, esp, vrrp, udp,   or tcp. Note that the identifiers tcp, udp,   and icmp are also keywords and must be   escaped via backslash (\), which is \\ in   the C-shell. Note that this primitive does   not chase the protocol header chain.  ip6 proto protocol   True if the packet is an IPv6 packet of pro-   tocol type protocol. Note that this primi-   tive does not chase the protocol header   chain.  ip6 protochain protocol   True if the packet is IPv6 packet, and con-   tains protocol header with type protocol in   its protocol header chain. For example,    ip6 protochain 6   matches any IPv6 packet with TCP protocol   header in the protocol header chain. The   packet may contain, for example, authentica-   tion header, routing header, or hop-by-hop   option header, between IPv6 header and TCP   header. The BPF code emitted by this primi-   tive is complex and cannot be optimized by   BPF optimizer code in tcpdump, so this can   be somewhat slow.  ip protochain protocol   Equivalent to ip6 protochain protocol, but   True if the packet is an ethernet broadcast   packet. The ether keyword is optional.  ip broadcast   True if the packet is an IP broadcast   packet. It checks for both the all-zeroes   and all-ones broadcast conventions, and   looks up the local subnet mask.  ether multicast   True if the packet is an ethernet multicast   packet. The ether keyword is optional.   This is shorthand for ‘ether[0] & 1 != 0’.  ip multicast   True if the packet is an IP multicast   packet.  ip6 multicast   True if the packet is an IPv6 multicast   packet.  ether proto protocol   True if the packet is of ether type proto-   col. Protocol can be a number or one of the   names ip, ip6, arp, rarp, atalk, aarp, dec   net, sca, lat, mopdl, moprc, iso, stp, ipx,   or netbeui. Note these identifiers are also   keywords and must be escaped via backslash   (\).   [In the case of FDDI (e.g., ‘fddi protocol   arp’) and Token Ring (e.g., ‘tr protocol   arp’), for most of those protocols, the pro-   tocol identification comes from the 802.2   Logical Link Control (LLC) header, which is   usually layered on top of the FDDI or Token   Ring header.   When filtering for most protocol identifiers   on FDDI or Token Ring, tcpdump checks only   the protocol ID field of an LLC header in   so-called SNAP format with an Organizational   Unit Identifier (OUI) of 0x000000, for   encapsulated Ethernet; it doesn't check   whether the packet is in SNAP format with an   OUI of 0x000000.   The exceptions are iso, for which it checks   the DSAP (Destination Service Access Point)   and SSAP (Source Service Access Point)   fields of the LLC header, stp and netbeui,   packet with an OUT of 0x080007 and the   Appletalk etype.   In the case of Ethernet, tcpdump checks the   Ethernet type field for most of those proto-   cols; the exceptions are iso, sap, and net   beui, for which it checks for an 802.3 frame   and then checks the LLC header as it does   for FDDT and Token Ring, atalk, where it   checks both for the Appletalk etype in an   Ethernet frame and for a SNAP-format packet   as it does for FDDT and Token Ring, aarp,   where it checks for the Appletalk ARP etype   in either an Ethernet frame or an 802.2 SNAP   frame with an OUT of 0x000000, and ipx,   where it checks for the IPX etype in an Eth-   ernet frame, the IPX DSAP in the LLC header,   the 802.3 with no LLC header encapsulation   of IPX, and the IPX etype in a SNAP frame.]  decnet src host   True if the DECNET source address is host,   which may be an address of the form   “10.123” , or a DECNET host name. [DECNET   host name support is only available on   Ultrix systems that are configured to run   DECNET.]  decnet dst host   True if the DECNET destination address is   host.  decnet host host   True if either the DECNET source or destina-   tion address is host.  ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp,   ipx, netbeui   Abbreviations for:    ether proto p   where p is one of the above protocols.  lat, moprc, mopdl   Abbreviations for:    ether proto p   where p is one of the above protocols. Note   that tcpdump does not currently know how to   parse these protocols.  vlan [vlan_Id]   True if the packet is an IEEE 802.1Q VLAN   packet. If [vlan_id] is specified, only   encountered in expression changes the decod-   ing offsets for the remainder of expression   on the assumption that the packet is a VLAN   packet.  tcp, udp, icmp   Abbreviations for:    ip proto p or ip6 proto p   where p is one of the above protocols.  iso proto protocol   True if the packet is an OSI packet of pro-   tocol type protocol. Protocol can be a num-   ber or one of the names clnp, esis, or isis.  clnp, esis, isis   Abbreviations for:    iso proto p   where p is one of the above protocols. Note   that tcpdump does an incomplete job of pars-   ing these protocols.  expr relop expr   True if the relation holds, where relop is   one of >, <, >=, <=, =, !=, and expr is an   arithmetic expression composed of integer   constants (expressed in standard C syntax),   the normal binary operators [+, −, *, /, &,   |], a length operator, and special packet   data accessors. To access data inside the   packet, use the following syntax:    proto [ expr : size ]   Proto is one of ether, fddi, tr, ip, arp,   rarp, tcp, udp, icmp or ip6, and indicates   the protocol layer for the index operation.   Note that tcp, udp and other upper-layer   protocol types only apply to IPv4, not IPv6   (this will be fixed in the future) . The   byte offset, relative to the indicated pro-   tocol layer, is given by expr. Size is   optional and indicates the number of bytes   in the field of interest; it can be either   one, two, or four, and defaults to one. The   length operator, indicated by the keyword   len, gives the length of the packet.   For example, ‘ether[0] & 1 != 0’ catches all   multicast traffic. The expression ‘ip[0] &   0xf != 5’ catches all IP packets with   options. The expression ‘ip[6:2] & 0x1fff =   0’ catches only unfragmented datagrams and   frag zero of fragmented datagrams. This   always means the first byte of the TCP   header, and never means the first byte of an   intervening fragment.   Some offsets and field values may be   expressed as names rather than as numeric   values. The following protocol header field   offsets are available: icmptype (TCMP type   field), icmpcode (TCMP code field), and   tcpflags (TCP flags field).   The following TCMP type field values are   available: icmp-echoreply, icmp-unreach,   icmp-sourcequench, icmp-redirect, icmp-echo,   icmp-routeradvert, icmp-routersolicit, icmp-   timxceed, icmp-paramprob, icmp-tstamp, icmp-   tstampreply, icmp-ireq, icmp-ireqreply,   icmp-maskreq, icmp-maskreply.   The following TCP flags field values are   available: tcp-fin, tcp-syn, tcp-rst, tcp-   push, tcp-push, tcp-ack, tcp-urg.  Primitives may be combined using:   A parenthesized group of primitives and   operators (parentheses are special to the   Shell and must be escaped).   Negation (‘!’ or ‘not’).   Concatenation (‘&&’ or ‘and’).   Alternation (‘∥’ or ‘or’).  Negation has highest precedence. Alternation and  concatenation have equal precedence and associate  left to right. Note that explicit and tokens, not  juxtaposition, are now required for concatenation.  If an identifier is given without a keyword, the  most recent keyword is assumed. For example,   not host vs and ace  is short for   not host vs and host ace  which should not be confused with   not ( host vs or ace )  Expression arguments can be passed to tcpdump as  either a single argument or as multiple arguments,  whichever is more convenient. Generally, if the  expression contains Shell metacharacters, it is  before being parsed.EXAMPLES To print all packets arriving at or departing from sun down:  tcpdump host sundown To print traffic between helios and either hot or ace:  tcpdump host helios and \( hot or ace \) To print all IP packets between ace and any host except helios:  tcpdump ip host ace and not helios To print all traffic between local hosts and hosts at Berkeley:  tcpdump net ucb-ether To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):  tcpdump ‘gateway snup and (port ftp or ftp-data)’ To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff should never make it onto your local net).  tcpdump ip and not net localnet To print the start and end packets (the SYN and FIN pack- ets) of each TCP conversation that involves a non-local host.  tcpdump ‘tcp[tcpflags] & (tcp-synltcp-fin) 0 and not srcand dst net localnet’ To print IP packets longer than 576 bytes sent through gateway snup:  tcpdump ‘gateway snup and ip[2:2] > 576’ To print IP broadcast or multicast packets that were not sent via ethernet broadcast or multicast:  tcpdump ‘ether[0] & 1 = 0 and ip[16] >= 224’ To print all ICMP packets that are not echo requests/replies (i.e., not ping packets);  tcpdump ‘icmp[icmptype] != icmp-echo and icmp[icmptype] !=icmp-echoreply’OUTPUT FORMAT The output of tcpdump is protocol dependent. The follow- ing gives a brief description and examples of most of the formats. Link Level Headers addresses, protocol, and packet length are printed. On FDDI networks, the ‘-e’ option causes tcpdump to print the ‘frame control’ field, the source and destination addresses, and the packet length. (The ‘frame control’ field governs the interpretation of the rest of the packet. Normal packets (such as those containing IP data grams) are ‘async’packets, with a priority value between 0 and 7; for example, ‘async4’. Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet. On Token Ring networks, the ‘-e’ option causes tcpdump to print the ‘access control’ and ‘frame control’ fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet. Regardless of whether the ‘-e’ option is speci- fied or not, the source routing information is printed for source-routed packets. (N.B. The following description assumes familiarity with the SLIP compression algorithm described in RFC-1144.) On SLIP links, a direction indicator (“I” for inbound, “O” for outbound), packet type, and compression informa- tion are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp. No further link information is printed for ip packets. For TCP packets, the connection identifier is printed following the type. If the packet is compressed, its encoded header is printed out. The special cases are printed out as *S+n and *SA+n, where n is the amount by which the sequence number (or sequence number and ack) has changed. If it is not a spe- cial case, zero or more changes are printed. A change is indicated by U (urgent pointer), W (window), A (ack), S (sequence number), and I (packet ID), followed by a delta (+n or −n), or a new value (=n) . Finally, the amount of data in the packet and compressed header length are printed. For example, the following line shows an outbound com- pressed TCP packet, with an implicit connection identi- fier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header:  O ctcp * A+6 S+49 I+6 3 (6) ARP/RARP Packets Arp/rarp output shows the type of request and its argu- ments. The format is intended to be self explanatory.  arp who-has csam tell rtsg  arp reply csam is-at CSAM The first line says that rtsg sent an arp packet asking for the ethernet address of internet host csam. Csam replies with its ethernet address (in this example, ether net addresses are in caps and internet addresses in lower case). This would look less redundant if we had done tcpdump -n:  arp who-has 128.3.254.6 tell 128.3.254.68  arp reply 128.3.254.6 is-at 02:07:01:00:01:c4 If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be visible:  RTSG Broadcast 0806 64: arp who-has csam tell rtsg  CSAM RTSG 0806 64: arp reply csam is-at CSAM For the first packet this says the ethernet source address is RTSG, the destination is the ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes. TCP Packets (N.B.:The following description assumes familiarity with the TCP protocol described in RFC-793. If you are not familiar with the protocol, neither this description nor tcpdump will be of much use to you.) The general format of a tcp protocol line is:  src > dst: flags data-seqno ack window urgent options Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single +1 .’ (no flags). Data-seqno describes the portion of sequence space covered by the data in this packet (see example below) . Ack is sequence number of the next data expected the other direc- tion on this connection. Window is the number of bytes of receive buffer space available the other direction on this connection. Urg indicates there is ‘urgent’ data in the packet. Options are tcp options enclosed in angle brack- ets (e.g., <mss 1024>). Src, dst and flags are always present. The other fields depend on the contents of the packet's tcp protocol header and are output only if appropriate. Here is the opening portion of an rlogin from host rtsg to host csam.  rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss1024>  csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win4096 <mss 1024>  rtsg.1023 > csam.login: . ack 1 win 4096  rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096  csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077  csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1  csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1 The first line says that tcp port 1023 on rtsg sent a packet to port login on csam. The S indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. (The notation is ‘first:last(nbytes)’ which means ‘sequence numbers first up to but not including last which is nbytes bytes of user data’.) There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment- size option requesting an mss of 1024 bytes. Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN. Rtsg then acks csam's SYN. The ‘.’ means no flags were set. The packet con- tained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1) . The first time tcpdump sees a tcp ‘conversation’, it prints the sequence number from the packet. On subsequent pack ets of the conversation, the difference between the cur- rent packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data stream (with the first data byte each direction being ‘1’). ‘-S’ will override this fea- ture, causing the original sequence numbers to be output. On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the rtsg -> csam side of the conversation) The PUSH flag is set in the packet. On the 7th line, csam says it's received data sent by rtsg up to but not includ- ing byte 21. Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg. If the snapshot was small enough that tcpdump didn't cap- ture the full TCP header, it interprets as much of the header as it can and then reports “[|tcp]” to indicate the remainder could not be interpreted. If the header contains a bogus option (one with a length that's either too small or beyond the end of the header), tcpdump reports it as “[bad opt]” and does not interpret any further options (since it's impossible to tell where they start) . If the header length indicates options are pre- sent but the IP datagram length is not long enough for the options to actually be there, tcpdump reports it as “[bad hdr length]” There are 8 bits in the control bits section of the TCP header:  CWR | ECE | URG | ACK | ESH | RST | SYN | FIN Let's assume that we want to watch packets used in estab- lishing a TCP connection. Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is  1) Caller sends SYN  2) Recipient responds with SYN, ACK  3) Caller sends ACK Now we're interested in capturing packets that have only the SYN bit set (Step 1) . Note that we don't want packets from step 2 (SYN-ACK), just a plain initial SYN. What we need is a correct filter expression for tcpdump. Recall the structure of a TCP header without options:   A TCP header usually holds 20 octets of data, unless options are present. The first line of the graph contains octets 0-3, the second line shows octets 4-7 etc. Starting to count with 0, the relevant TCP control bits are contained in octet 13:   Let's have a closer look at octet no. 13:   These are the TCP control bits we are interested in. We have numbered the bits in this octet from 0 to 7, right to left, so the ESH bit is bit number 3, while the URG bit is number 5. Recall that we want to capture packets with only SYN set. Let's see what happens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:   Looking at the control bits section we see that only bit number 1 (SYN) is set. Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is  00000010 and its decimal representation is   We're almost done, because now we know that if only SYN is set, the value of the 13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte order, must be exactly 2. This relationship can be expressed as  tcp[13] == 2 We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set:  tcpdump −i x10 tcp[13] == 2 The expression says “let the 13th octet of a TCP datagram have the decimal value 2”, which is exactly what we want. Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:   Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is   00010010 which translates to decimal   Now we can't just use ‘tcp[13] == 18’ in the tcpdump fil- ter expression, because that would select only those pack- ets that have SYN-ACK set, but not those with only SYN set. Remember that we don't care if ACK or any other con- trol bit is set as long as SYN is set. In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN:   We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true:  ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 ) This points us to the tcpdump filter expression   tcpdump −i x10 ‘tcp[13] & 2 == 2’ Note that you should use single quotes or a backslash in the expression to hide the AND (‘&’) special character from the shell. UDE Packets UDP format is illustrated by this rwho packet:  actinide.who > broadcast.who: udp 84 This says that port who on host actinide sent a udp data Some UDP services are recognized (from the source or des- tination port number) and the higher level protocol infor- mation printed. In particular, Domain Name service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS. UDP Name Server Requests (N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC-1035. If you are not familiar with the protocol, the following descrip- tion will appear to be written in greek.) Name server requests are formatted as  src > dst: id op? flags qtype qclass name (len)  h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu.(37) Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucb vax.berkeley.edu. The query id was ‘3’. The ‘+’ indi- cates the recursion desired flag was set. The query length was 37 bytes, not including the UDP and IP protocol headers. The query operation was the normal one, Query, so the op field was omitted. If the op had been anything else, it would have been printed between the ‘3’ and the Similarly, the qclass was the normal one, C_IN, and omitted. Any other qclass would have been printed immedi- ately after the ‘A’. A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as ‘[na] ’, ‘[nn]’ or ‘[nau]’ where n is the appropriate count. If any of the response bits are set (AA, PA or rcode) or any of the ‘must be zero’ bits are set in bytes two and three, ‘[b2&3=x]’ is printed, where x is the hex value of header bytes two and three. UDP Name Server Responses Name server responses are formatted as  src > dst: id op rcode flags a/n/au type class data (len)  helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)  helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97) In the first example, hellos responds to query id 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is internet address 128.32.137.3. The total size of the response was 273 bytes, excluding UDP and IP headers. The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the A response code of non-existent domain (NXDomain) with no answers, one name server and no authority records. The ‘*’ indicates that the authoritative answer bit was set. Since there were no answers, no type, class or data were printed. Other flag characters that might appear are ‘-’ (recursion available, RA, not set) and ‘|’ (truncated message, TC, set) . If the ‘question’ section doesn't contain exactly one entry, ‘[nq]’ is printed. Note that name server requests and responses tend to be large and the default snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag to increase the snaplen if you need to seriously investigate name server traffic. ‘-s 128’ has worked well for me. SMB/CIFS decoding tcpdump now includes fairly extensive SMB/CIFS/NBT decod- ing for data on UDP/137, UDP/138 and TCP/139. Some primi- tive decoding of IPX and NetBEUI SMB data is also done. By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory details. If you are decoding SMB sessions containing unicode strings then you may wish to set the environment variable USE_UNICODE to 1. A patch to auto-detect unicode srings would be welcome. For information on SMB packet formats and what all te fields mean see www.cifs.org or the pub/samba/specs/ directory on your favourite samba.org mirror site. The SMB patches were written by Andrew Tridgell (tridge@ samba.org). NFS Requests and Replies Sun NFS (Network File System) requests and replies are printed as:  src.xid > dst.nfs: len op args  src.nfs > dst.xid: reply stat len op results  sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165  wrl.nfs > sushi.6709: reply ok 40 readlink “. . . /var”  sushi.201b > wrl.nfs:   144 lookup fh 9,74/4096.6878 “xcolors” In the first line, host sushi sends a transaction with id 6709 to wrl (note that the number following the src host is a transaction id, not the source port) . The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file han- dle (fh) 21,24/10.731657119. (If one is lucky, as in this case, the file handle can be interpreted as a major, minor device number pair, followed by the mode number and gen- eration number.) Wrl replies ‘ok’ with the contents of the link. In the third line, sushi asks wrl to lookup the name ‘xcolors’ in directory file 9,74/4096.6878. Note that the data printed depends on the operation type. The format is intended to be self explanatory if read in conjunction with an NFS protocol spec. If the -v (verbose) flag is given, additional information is printed. For example:  sushi.1372a > wrl.nfs:   148 read fh 21,11/12.195 8192 bytes @ 24576  wrl.nfs > sushi.1372a:   reply ok 1472 read REG 100664 ids 417/0 sz 29388 (-v also prints the IP header TTL, ID, length, and frag- mentation fields, which have been omitted from this exam- ple.) In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl replies ‘ok’; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP headers and so might not be printed, depending on the filter expression used) . Because the -v flag is given, some of the file attributes (which are returned in addition to the file data) are printed: the file type (“REG”, for regu- lar file), the file mode (in octal), the uid and gid, and the file size. If the -v flag is given more than once, even more details are printed. Note that NFS requests are very large and much of the detail won't be printed unless snaplen is increased. Try using ‘-s 192’ to watch NFS traffic. NFS reply packets do not explicitly identify the RPC oper- ation. Instead, tcpdump keeps track of “recent” requests, and matches them to the replies using the trans- action ID. If a reply does not closely follow the corre Transarc AFS (Andrew File System) requests and replies are printed as:  src.sport > dst.dport: rx packet-type  src.sport > dst.dport: rx packet-type service call call-name args  src.sport > dst.dport: rx packet-type service reply call-name args  elvis.7001 > pike.afsfs:   rx data fs call rename old fid 536876964/1/1“.newsrc.new”   new fid 536876964/1/1 “.newsrc”  pike.afsfs > elvis.7001: rx data fs reply rename In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of 536876964/1/1 and an old filename of ‘.newsrc.new’, and a new directory file id of 536876964/1/1 and a new filename of ‘.newsrc’. The host pike responds with a RPC reply to the rename call (which was successful, because it was a data packet and not an abort packet). In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the ‘interesting’ arguments, for some definition of interesting). The format is intended to be self-describing, but it will probably not be useful to people who are not familiar with the workings of AFS and RX. If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is printed, such as the the RX call ID, call number, sequence number, serial number, and the RX packet flags. If the -v flag is given twice, additional information is printed, such as the the RX call ID, serial number, and the RX packet flags. The MTU negotiation information is also printed from RX ack packets. If the -v flag is given three times, the security index and service Id are printed. Error codes are printed for abort packets, with the excep- tion of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol). Note that AFS requests are very large and many of the arguments won't be printed unless snaplen is increased. Try using ‘-s 256’ to watch AFS traffic. ation. Instead, tcpdump keeps track of “recent” requests, and matches them to the replies using the call number and service ID. If a reply does not closely follow the corresponding request, it might not be parsable. KIP Appletalk (DDE in UDP) Appletalk DDE packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDE packets (i.e., all the UDE header information is discarded). The file /etc/atalk.names is used to translate appletalk net and node numbers to names. Lines in this file have the form  numbername   1.254ether  16.1icsd-net   1.254.110ace The first two lines give the names of appletalk networks. The third line gives the name of a particular host (a host is distinguished from a net by the 3rd octet in the number - a net number must have two octets and a host number must have three octets.) The number and name should be sepa- rated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a ‘#’). Appletalk addresses are printed in the form  net.host.port  144.1.209.2 > icsdnet.112.220  office.2 > icsd-net.112.220  jssmag.149.235 > icsd-net.2 (If the /etc/atalk.names doesn't exist or doesn't contain an entry for some appletalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDE port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (‘office’). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBE port (note that the broadcast address (255) is mdi cated by a net name with no host number - for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names). NBP (name binding protocol) and ATE (Appletalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size. NBP packets are formatted like the following examples:  techpit.2 > icsd-net.112.220: nbp-reply 190:“techpit:LaserWriter@*”186 The first line is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp id for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named “RM1140” registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter “techpit” registered on port 186. ATE packet formatting is demonstrated by the following example:  jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae03000l  helios.132 > jssmag.209.165: atp-resp 12266:0 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:1 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:2 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:3 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:4 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:5 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:6 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp*12266:7 (512)0xae040000  jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae03000l  helios.132 > jssmag.209.165: atp-resp 12266:3 (512)0xae040000  helios.132 > jssmag.209.165: atp-resp 12266:5 (512)0xae040000  jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae03000l  jssmag.209.133 > helios.132: atp~req* 12267<0-7> 0xae030002 Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the ‘<0-7>’). The hex num- ber at the end of the line is the value of the ‘userdata’ field in the request. Helios responds with 8 512-byte packets. The ‘:digit’ following the transaction id gives the packet sequence number in the transaction and the number in parens is the amount of data in the packet, excluding the atp header. The ‘*’ on packet 7 indicates that the FOM bit was set. Jssmag.209 then requests that packets 3 & 5 be retransmit- ted. Helios resends them then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The ‘*’ on the request indicates that XO (‘exactly once’) was not set. IP Fragmentation Fragmented Internet datagrams are printed as  (frag id:size@offset+)  (frag id:size@offset) (The first form indicates there are more fragments. The second indicates this is the last fragment.) offset (in bytes) in the original datagram. The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connec- tion that doesn't appear to handle 576 byte datagrams:  arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win4096 (frag 595a:328@0+)  arizona > rtsg: (frag 595a:204@328)  rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560 There are a couple of things to note here: First, addresses in the 2nd line don't include port numbers. This is because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you. A packet with the IP don't fragment flag is marked with a trailing (DF). Timestamps By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form  hh:mm:ss.frac and is as accurate as the kernel's clock. The timestamp reflects the time the kernel first saw the packet. No attempt is made to account for the time lag between when the ethernet interface removed the packet from the wire and when the kernel serviced the ‘new packet’ interrupt.SEE ALSO traffic(1C), nit(4P), bpf(4), pcap(3)AUTHORS The original authors are: Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of Cali- fornia, Berkeley, CA. It is currently being maintained by tcpdump.org. The current version is available via http: The original distribution is available via anonymous ftp:  ftp://ftp.ee.lbl.gov/tcpdump.tar.Z IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young's SSLeay library, under specific configuration.BUGS Please send problems, bugs, questions, desirable enhance- ments, etc. to:  tcpdump-workers@tcpdump.org Please send source code contributions, etc. to:  patches@tcpdump.org NIT doesn't let you watch your own outbound traffic, BPF will. We recommend that you use the latter. On Linux systems with 2.0[.x] kernels:  packets on the loopback device will be seen twice;  packet filtering cannot be done in the kernel, so  that all packets must be copied from the kernel in  order to be filtered in user mode;  all of a packet, not just the part that's within  the snapshot length, will be copied from the kernel  (the 2.0[.x] packet capture mechanism, if asked to  copy only part of a packet to userland, will not  report the true length of the packet; this would  cause most IP packets to get an error from tcp  dump). We recommend that you upgrade to a 2.2 or later kernel. Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol. Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program gener- ating them rather than tcpdump. A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored) headers assume that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet packets. This is true for IP, ARP, and DECNET Phase IV, but is not true for protocols such as ISO CLNS. Therefore, the filter may inadvertently accept certain packets that do not properly match the fil- ter expression. Filter expressions on fields other than those that manipu- late Token Ring headers will not correctly handle source- routed Token Ring packets. ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behavior. Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only looks at IPv4 packets.
(end man page)